a) Field of the Invention
The present invention relates to an illumination system used for optical instruments such as microscopes, specifically to that assuring an optimum numerical aperture and bright and uniform illumination light over a predetermined area, upon characteristics of a diffusing surface (diffusing plate) disposed in a path of light being adjustable by means of holographic techniques or the like.
b) Description of the Prior Art
There is art for varying diffusion characteristics on a diffusing surface, portion by portion by application of holographic technique, as described in U.S. Pat. Nos. 5,046,793 and 5,418,631, "Holographic diffuser", Wadle et al., Opt.Eng.33,213-218 (1994), etc.
On the other hand, there is other art using a fly eye lens for attaining uniform brightness of illumination, as generally applied to steppers. Specifically, Japanese Preliminary Publication No. Sho 59-111,124 discloses an example of this art applied to a microscope.
Where light from a diffusing plate is used as illumination light via a lens, the principal diffusion direction and the diffusion range of the diffused light should be optimized on the diffusing plate, portion by portion, for attaining an ideal numerical aperture and uniform and bright illumination light over a predetermined area. However, this is not feasible with an ordinary diffusing plate. For example, in a microscope optical system where a Koehler illumination system is applied so that an image of a light source is projected on a pupil of a condenser optical system, non-uniform light emission of the light source caused by a filament is dissolved by a configuration in which a diffusing plate is disposed between a collector optical system and a condenser optical system so that non-uniformity of illumination on a specimen surface is stayed as much as possible and so that light rays fill a sufficient area of the pupil. However, in practice, since diffusion characteristics on the diffusing plate cannot be controlled in a fine manner, if the degree of diffusion is set so high as to give a satisfactory level of both of the removal of illumination non-uniformity on the specimen surface and the occupation of the pupil by light rays, loss of amount of light caused by such a high degree of diffusion renders the visual field of the microscope dark. Therefore, the conventional microscope optical system keeps a balance between these requirements upon degree of diffusion being more or less moderated.
On the other hand, holographic techniques may be applied to a diffusing surface for the purpose of changing diffusion characteristics thereon, portion by portion, as described in the aforementioned U.S. Pat. No. 5,418,631, which is configured such that principal diffusion directions of diffused light at the diffusing surface are uniform despite that incident angles of the corresponding incident light differ, portion by portion, on the diffusing surface because light rays that emerge from the diffusing plate are to directly illuminate an object. Consequently, the principal diffusion direction is not varied, portion by portion, on the diffusing surface. Furthermore, U.S. Pat. No. 5,418,631 does not mention the numerical aperture for illumination light, because optical elements such as lenses are not disposed after the diffusing surface, i.e. illumination light involving a large numerical aperture is not considered.
In the optical system disclosed by U.S. Pat. No. 5,046,793, the principal diffusion direction of the diffused light is varied, portion by portion, on the diffusing surface, because the optical system is used for a display screen. However, uniformity of illumination light or the numerical aperture for illumination light are not mentioned.
"Holographic diffuser", Wadle et al. Opt. Eng. 33,213-218 (1994) describes a technique for adjusting angular range of diffusion, but does not mention how the result affects the uniformity of the illumination light or the numerical aperture for the illumination light.
As for the approach using a fly eye lens for obtaining uniform illumination, if an extremely high uniformity of illumination with deviation not greater than several percent is required, as in the case of a stepper, an optical system including a unit of two lens surfaces is employed. Such an approach involves a more complex structure than a method according to which uniform illumination light is obtained by a single diffusing plate, and sometimes needs a certain adjustment device. These features are disadvantageous.
Where the requirement for illumination uniformity is not so severe as in the stepper, there is art in which a fly eye lens forms multiple filament images as a secondary light source spreading in a space for the purposes of assuaring a certain numerical aperture for illumination light and reducing non-uniformity of illumination brightness, as shown, for example, in Japanese Patent Preliminary Publication No. Sho 59-111,124. However, in practice, since it is impossible to reduce the size of each lens element of the fly eye lens to infinity, this approach involves a defect in that borders between the lens elements or the plurality of filament images are directly visible at the pupil.
To overcome this defect, the illumination system disclosed by Sho 59-111,124 is configured such that a diffusing plate having a very small diffusing effect is disposed after the fly eye lens. In this case also, the structure becomes more complex than a configuration in which a diffusing plate is used alone.